This invention relates to a pressure-resistant buoyancy material to be used mainly under water of great depth (hereinafter referred to as "deep water").
In recent years, efforts are being continued for the development of techniques for lowering various observation instruments to great depths in the sea, operating them under water, and later raising them to the surface. Such techniques are required for deep water surveys conducted by submarines for economic and academic purposes. The use of such instruments in the manner described above necessitates a pressure-resistant buoyancy material of low specific gravity and high enough strength to withstand severe working conditions under deep water.
As pressure-resistant buoyancy materials capable of producing ample buoyancy under deep water, hollow plastic spheres, hollow glass spheres, syntactic foam compositions, etc. have been in use.
Use of hollow spheres made of metallic material is conceivable. These hollow spheres, however, are not suitable as a buoyancy material because they have a large specific gravity and low buoyancy.
Among commercially available hollow plastic sphere buoyancy materials is a product which is effectively usable under water at depths up to 1500 meters (manufactured by Ube Resin Processing Co., Ltd. and marketed under trademark designation of "Cycolac Flote"). This hollow plastic sphere is made of ABS resin (compression strength 480 kg/cm.sup.2) which measures 360 mm in diameter, weighs 10 kg, and has a specific gravity of 0.41.
Among commercially available hollow glass sphere buoyancy materials is a product of Benthos Inc. in the United States, which measures 432 mm in diameter, weighs 17.7 kg, and has a specific gravity of 0.42 and a working water depth of 6000 m. The most serious drawback suffered by any hollow glass sphere resides in the fact that it is vulnerable to shocks.
As means of improving hollow glass spheres by eliminating this serious drawback, the inventor has so far developed a pressure-resistant buoyancy material formed of hollow ceramic spheres and a pressure-resistant buoyancy material formed of hollow ceramic spheres and syntactic foam composition (Japanese Patent Application SHO No. 58(1983)-204729).
The hollow ceramic sphere involved in the invention just mentioned is usable effectively as a buoyancy material under water of a greater depth than the conventional hollow plastic sphere and hollow glass sphere.
These hollow spheres, because of their peculiar shape, invariably necessitate special devices for effective attachment to submarines, which have only complicated contours available for contact with the spheres.
As a convenient buoyancy material for a submarine, therefore, a syntactic foam which is formed of hollow glass microspheres and polyester resin or epoxy resin has found acceptance. Methods for the production of such syntactic foam are disclosed in Japanese Patent Disclosure SHO No. 49(1974)-58162, U.S. Pat. No. 3,477,967, and Japanese Patent Disclosure SHO No. 57(1982)-28142, for example.
The syntactic foam is obtained by pouring a raw material, which is a mixture of hollow glass microspheres and thermosetting resin, in a mold and allowing it to set. Thus, the syntactic foam can be obtained in various shapes conforming exactly to the cavity of a given mold. It therefore proves advantageous for use with a submarine which necessitates a buoyancy material of complicated shape as mentioned above.
The properties of the latest syntactic foam published in the research report, JAMSTECTR 12 (1984), of the Ocean Science Technology Center are shown in the following table.
______________________________________ High-strength Low-specific type gravity type ______________________________________ Specific gravity 0.561 0.545 Compression strength (kgf/cm.sup.2) 920 867 Crushing strength (kgf/cm.sup.2) 1276 1238 ______________________________________
For any syntactic foam to withstand use under deep water of 6000 m, the compressive strength and the crushing strength are required to be about 900 kgf/cm.sup.2 and 1240 kgf/cm.sup.2 respectively, with the safety factor calculated as 2. The high-strength type shown in the table meets this requirement. The highest specific gravity obtained by the technique of the existing standard is approximately 0.56.
Today ocean surveys are required to be conducted at still greater depths. To meet the requirements, a need is felt for the development of a buoyancy material, specifically a syntactic foam, possessing lower specific gravity and higher strength.
For the purpose, it is considered to be necessary:
(1) to use hollow glass microspheres having lower specific gravity and higher strength, PA0 (2) to improve the packing factor (ratio of bulk to true particle density) of the hollow glass microspheres, and PA0 (3) to use resin of high strength.
In order to increase the strength of the hollow glass spheres it is necessary to use glass of high rigidity, which is incompatible with the aim of reducing specific gravity. The packing factor of the hollow glass microspheres can be increased by combining spheres of different diameters, but there is a limit to the degree of compactness that can be obtained. When the product of 3M Corp. marketed under the trademark "Glass Bubble F29x" is adopted and 100-micron and 30-micron grades of the product are mixed in a ratio of 60:40 (so that the average specific gravity of the hollow glass microspheres is about 0.28), for example, the packing factor is 73%. When the gaps separating the adjacent hollow microspheres are filled up with resin of a specific gravity of 1.2, then the produced syntactic foam has an overall specific gravity of 0.528. If the specific gravity of this resin is lowered to decrease the overall specific gravity of the syntactic foam, the syntactic foam itself has the strength thereof proportionally lowered. Where the specific gravity of the hollow microspheres and that of the resin used in the syntactic foam are lowered, then the produced syntactic foam has the strength thereof lowered consequently. It is not possible to effect the desired decrease of specific gravity without a sacrifice of the strength of the syntactic foam.
The inventor continued a study with a view to developing a buoyancy material of improved performance and consequently developed the aforementioned novel pressure-resistant buoyancy material formed of hollow ceramic microspheres and a syntactic foam. This buoyancy material has been filed for patent under Japanese Patent Application SHO No. 58(1983)-204729. As compared with the conventional syntactic foam, this buoyancy material permits further reduction of specific gravity and further increase of strength and is suitable for use under deep water. When the hollow ceramic spheres and the syntactic foam are combined in intimate mutual contact, the ratio of volume reduction under application of pressure differs between the two components of the buoyancy material because the ratio of voluminal elasticity is not equal between them. The pressure so applied, therefore, is liable to impair uniform distribution if stress and lower the overall strength of the buoyancy material.
The present invention is characterized by disposing an empty space in the boundary between the syntactic foam and the hollow ceramic spheres thereby eliminating the disadvantage suffered by the conventional buoyancy material formed of such two components.